1. Field of the Invention
This invention relates to a hot wire arc welding torch assembly having a high dielectric resistance characteristic.
2. Description of the Prior Art
A nonconsumable metal electrode and a heated filler metal are used in a hot wire arc welding system. Gas-shielded tungsten arc hot wire welding and plasma arc hot wire welding are typical examples of such a system. A gas-shielded tungsten arc forms a weld puddle on a workpiece, and a filler metal is continuously fed into the puddle to form a weld bead. An alternating current is applied through the filler metal, with the magnitude of the current adjusted to heat the filler metal to its melting point just as it reaches the weld. The filler metal will then form a melt line just above the surface of the puddle.
U.S. Pat. Nos. 3,122,629 and 3,163,743 disclose hot wire arc welding systems which increase the deposition rate of the filler metal as compared with using a non-heated filler metal. These systems require two relatively large torches, however, one for generating the arc and another for heating the filler metal, and are therefore not well suited for manual welding.
Accordingly, in order to enable manual welding it is necessary that both torches be combined and miniatured to the extent that the resulting assembly is easily manipulable. In the past it has been proposed that the torch for heating the filler metal be disposed in parallel with a tungsten inert gas (T.I.G.) torch, with the tip of the heating torch being bent toward the T.I.G. torch.
Referring to FIG. 1 and FIG. 2, a typical conventional hot wire arc welding torch assembly is illustrated, including a T.I.G. torch 1 whose body is molded from a heat-proof synthetic resin. A tungsten electrode 12 is held by a collet body 15, which is mounted in the torch 1 such that the electrode is coaxially disposed. A holder 2 fixes the T.I.G. torch to a torch 4 for heating a filler metal 6. As shown in FIG. 2, the holder has a pinch gap 16 clamped by a bolt 13. A shielding gas nozzle 3 supplies an arc 10 and a weld puddle 11 with inert gas, and thereby shields the welding zone from the atmosphere. A rotatable knob 9 on the heating torch 4 controls the position of the filler metal 6. A power source 8 supplies the filler metal with current which flows through the weld puddle 11 and a workpiece 7. The electrical power source for establishing the plasma arc 10 is conventional, and not shown in the interest of simplicity.
Such a conventional T.I.G. torch assembly has a generally tapered shape in the area at which the holder 2 is clamped on, and in use there is a tendency for the holder to expand due to the radiant heat generated during welding. As a result the holder 2 is prone to become loose and slip, whereby the target critical welding position of the filler metal 6 is disrupted; this produces a low quality weld. If the bolt 13 is tightened in order to avoid this problem the molded T.I.G. torch body can easily be cracked and destroyed. Also, since in T.I.G. welding the arc is established by high-frequency energy, when it is difficult to initiate the arc 10 between the electrode 12 and the workpiece 7, the molded body portion of the torch induces dielectric breakdown. This causes leakage of the high-frequency energy from a contact member (not shown) in the torch to the holder 2, and may result in an electrical shock accident.